1. Technical Field
The present invention relates to a color image forming apparatus, an image forming method, and a recording medium suitably applicable to a tandem system color printer and a color copying machine having photosensitive drums and an intermediate transfer belt for executing a color misalignment correction mode and a color MFP (Multi Function Peripheral) thereof.
2. Description of Prior Art
In recent years, a tandem system color printer and color copying machine and a color MFP thereof have been used often. This kind of color image forming apparatus, to maintain optimally the print quality (color reproducibility) of a color image, superimposes yellow (Y), magenta (M), cyan (C), and black (BK) for reproducing red, green, and blue of a document image on an intermediate transfer belt. To superimpose reproducibly each color of Y, M, C, and BK, in an image forming unit, it is essential to correct positively color misalignments (hereinafter, referred to as a color misalignment correction mode).
Regarding the color misalignment correction mode, a color misalignment detection mark (hereinafter, referred to as a registration mark) for position detection formed on the intermediate transfer belt or a conveying transfer belt is detected by a detection section (hereinafter, referred to as a registration sensor) for color misalignment detection such as a reflection sensor, and the color misalignment amounts of registration marks of the other colors to the reference color registration mark are calculated and are fed back to each image forming unit of Y, M, and C so as to eliminate the color misalignment amounts, and the laser writing timing is corrected, thus a good color image can be obtained.
In relation to an apparatus having such a color misalignment correction mode, in Japanese Unexamined Patent Application Publication 5-188697 (JPA5-188697), an image forming apparatus is disclosed. According to this image forming apparatus, regarding the color registration correction timing, the correcting section, according to the cumulative time measured from the point of time when the image forming apparatus is powered, so as to postpone slowly the correction process start interval, controls the correction process start timing. When the apparatus is structured like this, for color misalignments due to the environmental temperature, the image formation interruption time can be minimized.
Further, in Japanese Unexamined Patent Application Publication 8-305108 (JPA8-305108), an image forming apparatus and a registration correction method therefor are disclosed. According to the registration correction method, from the registration execution history, the necessity of execution of the color misalignment correction mode is decided sequentially. By use of such a method, there is no need to perform a useless correction.
Furthermore, according to the image forming apparatus disclosed in Japanese Unexamined Patent Application Publication 9-244332 (JPA 9-244332), there are two kinds of color registration correction timing modes available and from the difference between the temperature of the exposure section and another intra-apparatus temperature, the color misalignment correction mode is selected. When the apparatus is structured like this, an image displacement can be corrected at optimum timing.
Further, according to the image forming apparatus disclosed in Japanese Unexamined Patent Application Publication 2004-198946 (JPA2004-198946), registration pattern marks are formed between recording sheets and the color misalignment correction process is performed. When the apparatus is structured like this, the color misalignment can be corrected without causing downtime.
On the other hand, according to the image forming apparatus relating to the conventional example, there are the following problems imposed.
i. Conventionally, in consideration of changes in the fixing temperature, the number of fed sheets, and the time period from the preceding correction, there are found many cases in which a correction condition is decided as a set value, and the execution time of the next color misalignment correction process is decided, and they are set as fixed values. However, even in consideration of all the conditions, it is difficult to take a perfect correlation between the image misalignment amount and the time period. As mentioned above, whether or not to execute the color registration correction process is often decided by the intra-apparatus temperature rise and number of fed sheets. If the interval of the color registration correction process is set short, the productivity and toner consumption are influenced, so that the color registration correction process is limited.
ii. According to JPA5-188697, regarding the color registration correction timing, according to the cumulative time measured from the point of time when the image forming apparatus is powered, the correction process start interval is postponed slowly, though if the concerned interval is set long, there is a fear of reduction in the image quality.
iii. According to JPA8-305108, from the registration execution history, the necessity of execution of the color misalignment correction mode is decided sequentially, though if the color misalignment time does not come and the color misalignment correction mode must be executed, it is difficult to judge the necessity. A color misalignment occurrence cause is fluid and in the transition period between the preceding correction process and the present correction process, stable images are not always outputted at the preceding correction amount.
iv. According to JPA9-244332, from the difference between the temperature of the exposure section and another intra-apparatus temperature, the color misalignment correction mode is selected, though it is very difficult to confirm completely the effect of the mounting position of the temperature sensor and the time period from the point of time when the preceding correction process ends on an image by the concerned temperature sensor.
v. According to JPA2004-198946, registration pattern marks are formed on an endless belt corresponding to the intervals between recording sheets and the color misalignment process is performed. In this case, it is difficult to determine the appropriate timing of color registration correction mode since data of color misalignment amount usually deviate in large extent.
vi. By the way, as shown in FIG. 16, a method for forming images at both ends outside the image area Ia on an intermediate transfer belt 6 and during forming the images on the intermediate transfer belt 6, executing the color misalignment correction mode may be considered. According to this kind of outside-image pattern detection example, it is necessary to mount a photosensitive drum with a width W′ covering the image area and an intermediate transfer belt with a width equal to the width W′, thus it results in obstruction to reduction in cost of the image forming apparatus.
Therefore, the present invention was developed to solve the aforementioned problems and is intended to provide a color image forming apparatus, an image forming method, and a recording medium for shortening overall the color misalignment correction time compared with the conventional color misalignment correction mode and improving the productivity of the concerned apparatus.